Safety device for charging device of electrophotographic apparatus

ABSTRACT

A charging device for use in electrophotographic apparatus has at least one corona discharge wire which is suspended on a wire retaining member. When the corona discharge wire is broken, one end of the wire retaining member is brought into contact with a short circuit electrode provided on a frame body to form a closed current loop to the ground.

United States Patent [72] Inventors IlisashiSakamaki [56] References Cited fi r 'f z' T k 0 UNITED STATES PATENTS 0s lmasa mura, 0 yo; samu 1848152 3/1932 Bie ger 317/14 A [2H A l N0 igwggura, Toky l J p 1,859,377 5/1932 Boudian 3l'7/l4A '22] ff [970 2,146,498 2 1939 Bradley etaL. 317/l4A [45 patented Sap. 28.19. 3,374,348 3/1968 Ostcnscn 250/495 |73| Assignee Canon Kahushikl Kaisha Primary Examiner-J. D. Miller Tokyo, Japan Assistant ExaminerHarry E. Moose, Jr. [32] Priority June 9, I969 AlturneyWard, McElhannon, Brooks & Fitzpatrick [33] Japan [31] 44/53157 [54] SAFETY DEVICE FOR CHARGING DEVICE 0F ELECTROPIIOTOGRAPI-IIC APPARATUS 4Cla'ms5Drawmg ABSTRACT: A charging device for use in electmpholo- [52] u.s.c1 317/262 A, g p apparatus has at least one corona discharge i 250/495 ZC, 317/14 A which is suspended on a wire retaining member. When the [51] lm. Cl 603 15/02 Corona discharge wire is broken, one end of the wire ining [50] Field of Search 317/14 A, member i gh inw ntac wi h a short circuit electr de 262 A, 16; 200/79; 250/4956 C, 49.5 TC, 49.5 ZC; 337/32 provided on a frame body to form a closed current loop to the ground.

PATENTEU SEP28 I97! FIG. I h

FIG. 2

OUTPUT CURRENT (I IA) FIG. 4A

FIG. 4B

SAFETY DEVICE FOR CHARGING DEVICE OF ELECTROPIIO'IOGRAPIIIC APPARATUS The present invention relates to a charging device for use in eleetrophotographic apparatus and more particularly to a charging device of the type described above provided with a safety device simple in construction and reliable in prevention of an accident or damage caused by the breakage of a discharge wire.

In general, in the electrophotographic apparatus, a surface of an electrophotographic photosensitive member having a photoconductive layer must be uniformly imparted with a predetermined magnitude of electrostatic charge. In a charging device (a discharger) is generally disposed a corona discharge wire made of tungsten or stainless steel of about 0.1 mm. in diameter and in opposed relation with the surface of the photosensitive member. A high voltage is applied across an electrode upon which is placed the photosensitive member and the corona discharge wire, thereby producing the corona discharge so as to impart the electrostatic charge to the surface of the photosensitive member.

Since a high voltage is applied, the corona discharge wire is gradually corroded and deteriorated so that the tensile strength of the wire is so reduced as to cause the breakage. Furthermore, when the toner or the like is attached to the corona discharge wire, the corona discharge tends to be concentrated at one portion so that a spark discharge is produced, thereby causing the breakage of the wire. In some cases, the copying paper is wrapped around the charging device and brought into contact with the corona discharge wire, thus resulting in the breakage. In this case, between the photosensitive member and the broken wire the spark is produced so that not only the photosensitive member but also the charging device are damaged. In the worst case, the electrophotographic apparatus itself is damaged and an operator may suffer electric shock.

To eliminate these problems, it has been proposed to bias a corona discharge wire retaining member by a spring in such a manner that when the wire is broken, the retaining member is caused to move outwardly, thereby actuating a switch or the like so as to interrupt a low voltage source such as the primary of a transformer. In this way the charging device must be provided with a high voltage circuit for charging and a low voltage circuit for safeguarding the device. The voltage difference between the two circuits is such so that large space is required in the device and the fabrication cost becomes higher. Even when a quick-to-respond microswitch is used, a torque or the order of 5 to gr.-cm. is required for actuating this microswitch. Thus, a large tension is required for the wire. On the other hand, the discharge cannot be produced efficiently at a relatively low voltage unless the diameter of the corona discharge wire is made smaller (generally 0.0. to 0.08 mm.). Therefore, greater tension may not be applied to the wire because breakage will occur at once. Thus, fabrication has been difficult.

In view of the above, the primary object of the present invention is to provide a charging device having a safety device so as to prevent accident or damage when the breakage of a corona discharge discharge wire occurs.

Another object of the present invention is to provide a charging device having a safety device capable of interrupting immediately the high voltage current flow through a corona discharge wire when the breakage thereof occurs.

A further object of the present invention is to provide a charging device having a safety device of the type which when the breakage of a corona discharge wire occurs, the high voltage to the wire is immediately grounded, thereby interrupting the high voltage to the wire.

The present invention will become more apparent from the following description of the preferred embodiments thereof taken in conjunction with the accompanying drawing, in which:

FIG. I is a front view of a charging device for electrophotographic apparatus embodying the present invention in one form;

FIG. 2 is an electric circuit diagram of the device shown in FIG. 1;

FIG. 3 is a graphical diagram of the voltage-current characteristic curve of a constant voltage transformer; and

FIG. 4A and 4B are fragmentary front views each illustrating the relationship between a short-circuit electrode and a corona discharge wire retaining member in accordance with the present invention.

Referring to FIG. 1, corona discharge wires 2 are enclosed in a conductive shielding frame body I, made of, for example, stainless steel, aluminum, etc. The corona discharge wires have a uniform, suitable diameter and are made of noncorrosive materials such as stainless steels. Both ends of the corona discharge wires 2 are fixed to elastic retaining members 3, and 3 made of electrically conductive material. It should be noted that instead of the elastic retaining member 3 a nonelastic retaining member may be utilized. The retaining members 3, and 3 are in turn securely fixed to insulating sidewalls 4, and 4 at both ends of the shielding frame body 1. Short circuit electrodes 5 which are fixed to the insulating sidewall are grounded and the charging device is provided with a cord for connection with an electric power source.

Normally high voltage is applied to the corona discharge wires 2 so that the surface of an electrophotographic photosensitive member may be imparted with the electrostatic charge. However, the corona discharge wires 2 are deteriorated by the corona discharge and broken by spark discharges, wrapping of copying papers or for other reasons. When the broken corona discharge wires 2 are adjacent to or in contact with an inflammable substance such as a photosensitive member, the spark discharge occurs therebetween so that the inflammable substance starts to burn, thus causing a tire. When the broken corona discharge wires 2 are in contact with a metallic part, it may cause an electric shock to an operator.

According to the present invention, however, when the corona discharge wires 2 are broken, their retaining members 3, and 3 are brought into contact with the short circuit electrodes 5 by the elasticity of each retaining member 3,. In consequence, the high voltage application to the corona discharge wires 2 are automatically interrupted. That is, a circuit consisting of a high voltage power transformer 7, a high voltage feed line 4, the retaining member 3, and the short circuit elec trode 5 is closed. (See FIG. 2) In this case, when an ordinary type transformer is used as the transformer 27, a fuse on the input side thereof will be melted. When a constant voltage transformer is used, the voltage-current characteristic curve is shown in FIG. 3. That is, immediately after the wire is broken the output voltage becomes zero so that the high voltage application to the corona discharge wires 2 is interrupted.

FIG. 4A shows another embodiment of the present invention in which a pair of retaining members are mounted on a support 8. In this embodiment, the retaining members have no elasticity, so that the retaining members on which the respective corona discharge wire is suspended at each end thereof are urged to the sidewall 4, of the shielding frame body 1 by respective tension springs 9 and 10 which have suitable tensile strength.

FIG. 4B shows another embodiment of the present invention in which a single retaining member 3 is pivoted. The corona discharge wires suspended on both ends of the retaining member 3 are balanced in respect to each other in the normal condition. When one discharge wire is broken, one end of the retaining member 3 is brought into contact with a short circuit electrode 5 by tensile force of the other discharge wire.

According to the present invention, a very reliable safeguarding may be attained only by the provision of the short circuit electrode 5 in opposed relation with the wire retaining member 3. The safety device of the present invention may dispense with a switch, lead wires, etc. which are required in the conventional device. Furthermore, only small tension is required to extend the corona discharge wires 2 because it is sufficient to space the retaining electrode 3 from the short circuit electrode 5 by a very small distance.

What is claimed is: l. A charging device for use in an electrophotographic apparatus comprising at least one corona discharge wire,

shielding members disposed in spaced apart relation with said corona discharge wire,

shielding member retaining members,

at least one short circuit electrode disposed adjacent to the end of said corona discharge wire on the side of application of high voltage thereto,

electrically conductive corona discharge wire retaining member on said side of high voltage application adapted to retain elastically said corona discharge wire and to be brought into contact with said short circuit electrode when breakage of said corona discharge wire occurs, and

a cord connectable to an electric power source for applying the high voltage to said corona discharge wire.

2. A charging device as defined in claim 1 wherein said corona discharge retaining member is made of an elastic material.

3. A charging device as defined in claim 1 wherein said corona discharge retaining member is maintained by spring.

4. A charging device as defined in claim 1 wherein an even number of corona discharge wires are provided,

said corona discharge wire retaining member has a fulcrum point and said wire retaining member is pivoted to retain the corona discharge wires on both sides of said fulcrum point, and at least two short circuit electrodes are disposed adjacent to the free ends of said wire retaining member. 

1. A charging device for use in an electrophotographic apparatus comprising at least one corona discharge wire, shielding members disposed in spaced apart relation with said corona discharge wire, shielding member retaining members, at least one short circuit electrode disposed adjacent to the end of said corona discharge wire on the side of application of high voltage thereto, electrically conductive corona discharge wire retaining member on said side of high voltage application adapted to retain elastically said corona discharge wire and to be brought into contact with said short circuit electrode when breakage of said corona discharge wire occurs, and a cord connectable to an electric power source for applying the high voltage to said corona discharge wire.
 2. A charging device as defined in claim 1 wherein said corona discharge retaining member is made of an elastic material.
 3. A charging device as defined in claim 1 wherein said corona discharge retaining member is maintained by spring.
 4. A charging device as defined in claim 1 wherein an even number of corona discharge wires are provided, said corona discharge wire retaining member has a fulcrum point and said wire retaining member is pivoted to retain the corona discharge wires on both sides of said fulcrum point, and at least two short circuit electrodes are disposed adjacent to the free ends of said wire retaining member. 